The Reptiles
A Quick Review of Amphibia
• Advantages of the
amphibious lifestyle
– escape predators
– exploit ephemeral water
bodies
– utilize previously
unavailable -- and
unexploited -- resources.
Fill vacant niches
A Quick Review of Amphibia
The transition to land
PROBLEM
DESCRIPTION
Support
Water provides much Modification and
more buoyancy than strengthening of fins,
air
pelvic and pectoral
girdles, vert. column
Tail used for
Use modified fins, but
propulsion in water, still use undulating
but won’t work in air motion for walking
Gills don’t work in air Lungs
– surface area
problem
Gills would lead to
Lungs, eyelids
excessive water loss
Locomotion
Respiration
Dessication
SOLUTION
A Quick Review of Amphibia
The transition to land
PROBLEM
DESCRIPTION SOLUTION
Sensing
vibration
Lateral line won’t
work in air
Reproduction
Fertilization,
dessication problems
Prey capture
Fish could use
suction to help
capture prey; won’t
work in air
Modified
hyomandibula
transmits vibration to
fluid-filled inner ear
Reproduce in water
Tongue
Amphibians were able to exploit
some terrestrial resources, but…
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Reproductive constraints, among other things,
forces them to remain in the vicinity
of water
Early amphibians gave rise to a new group
evolved from that was able to lay eggs on
dry land
The Reptiles
Amniotic Egg (more later)
• Developed in protoreptiles and is found
in their descendentsbirds and mammals
– no larval stage; young
look like miniature
adults
– provide “aquatic
environment” within
the egg
– THE MOST IMPORTANT
DIFFERENCE BETWEEN
AMPHIBIANS AND
REPTILES
Fossilized eggs in nest
Class Reptilia
General Characteristics of
Reptilia
• Tetrapod
– means fourlimbed
– secondarily lost in
snakes
General Characteristics of Reptilia
• Skin made of scales derived from the
epidermis
– ancestors of reptiles had heavy, bony dermal
scales
– reptiles developed lighter, more flexible scales
made of keratin
– some still retain bony plates in skin (e.g.
alligators)
– skin dry, rather than moist. Modern reptiles
generally lose less water via skin than modern
amphibians
General Characteristics of Reptilia
• Heart with three
chambers
• Amphibians have same
system;
• Ventricles not
completely separated so
there is some mixing of
oxygenated and deoxygenated blood
• Does this place any
constraints on activity?
• Crocodilians have four
chambered heart
General Characteristics of Reptilia
• Ectothermy
– depend primarily on
environmental
sources of heat
– basking, seeking
shade, etc.
– specialized body
structures
– much debate about
dinosaurs, though
• vascularization of
bones
• ratio of fossils of
predatory vs
herbivorous dinosaurs
Dimetrodon (not a dinosaur)
General Characteristics of Reptilia
• Amniotic Egg
– appeared app. 310
mya, about 50
million years after
the first amphibians
– required the
evolution of internal
fertilization
– has three
membranes not
found in amphibian
eggs: amnion,
chorion, and allantois
Components of the Amniotic Egg
• Shell
– may be leathery and
flexible (as in lizards)
or calcified (as in
birds)
– provides mechanical
protection while
allowing for gas and
water exchange
• Albumin (egg white)
– protection against
mechanical damage
– provides a reservoir of
water and protein
Albumin
Components of the Amniotic Egg
• Yolk
– rich in lipids (fats);
energy supply for the
developing embryo
– eventually enclosed by
the yolk sac, which is
part of the developing
gut
– by the end of
development only a little
yolk remains and this is
absorbed before or
shortly after hatching
• Every structure thus far
is common to both
amniotic and nonamniotic eggs
Albumin
Components of the Amniotic Egg
• Amnion
– inner membrane
surrounding the
embryo in a
contained
environment of
water and salts
– acts as a water
reservoir
– aids in
osmoregulation
Albumin
Components of the Amniotic Egg
• Chorion
– protective outer
membrane
– surrounds all
embryonic
structures
•
•
•
•
•
embryo
yolk sac
albumin
allantois
amnion
– vascularized for
gas exchange
Albumin
Components of the Amniotic Egg
• Allantois
– outgrowth of
embryonic hindgut
– used to store
nitrogenous wastes
produced by the
embryo
– vascularized; aids in
respiration
– increased in size as
embryo grows
Albumin
Possible Steps in the Evolution
of the Amniotic Egg
• Development of a terrestrial habit by adult
• Initiation of internal fertilization
• Reduction in body size (many early amphibians were
large)
– terrestrially laid, non-amniotic eggs > 10 mm diameter
cannot exchange enough oxygen for respiration, may also
collapse under their own weight
– since egg size and adult body size are correlated, ancestors
of the first amniotes were likely to have been small
• Reduction in the number of eggs produced, with
increase in size and quantity of yolk in each egg
• Shortening and later elimination of the larval stage
• Laying of small non-amniotic egg on land
• Development of amniotic membranes
Advantages of Amniotic Eggs
• Storage of wastes
– keeps toxic waste products away from the
developing embryo
• Improved gas exchange
• Can be laid in terrestrial environments
• Since amniotic eggs are not as sensitive
to the size constraints of terrestrial,
non-amniotic eggs, they could allow for
the evolution of larger body size
Disadvantages of Amniotic Eggs
and Laying Eggs in a Terrestrial
Environment
• Air is a far more variable environment than
water
– temperature is more variable and changes more
rapidly
– moisture content
• Must have internal fertilization
• Usually requires more parental care than fish or
amphibians (e.g. alligators)
• Relatively expensive to produce and since more
energy is invested in each egg, fewer eggs can
be produced by any individual female
Example: American Alligator
• Native to SE U.S.,
with large populations
in FL, LA, and parts of
GA and SC
• Large reptile that lays
amniotic eggs
– adult males average
11.2 feet in length
– adult females
average 8.2 feet
– but many get larger,
with records > 19 ft
and 1,000 lbs
Alligator Nests
• Lay eggs terrestrially
• Nest is typically on the
edge of a lake or river
• Constructed from
aquatic vegetation
Building
– keeps eggs above water
level
– rotting vegetation keeps
eggs warm, moist
• Since amniotic eggs can
“drown”, female must
predict future water
levels
• Usually 35 - 50 eggs are
laid; take 65 days to
hatch
Laying
Eggs
Hatching
1
• Female guards nest until
hatch
• As eggs begin to hatch,
nestlings make grunting
noises
• Female removes
vegetation to release
hatching young
• Female may carry young
to water’s edge in mouth
2
3
4
Early Life
• Female attends
young for a year or
more, protecting
them from predators
such as wading
birds or other
alligators
Pod of young alligators;
female is nearby
Cannibalism: adult male
eating a juvenile